1. Field of the Invention
The present invention relates to an electrophotographic [type] printing apparatus, and specifically to an electrophotographic printing apparatus by which dropout color printing can be determined.
2. Description of the Related Art
As a recording form in a printing apparatus, a method using a so-called electrophotographic process comes into general use, in which a photoreceptor surface is charged to a predetermined voltage, image exposure is conducted on the charged photoreceptor and an electrostatic latent image is formed, powder toner is supplied to the electrostatic latent image and a toner image is obtained on the photoreceptor surface, and this toner image is transferred onto the recording medium such as a sheet, an OHP sheet, or an intermediate transfer body.
As the developing system of this kind of electrophotographic printing apparatus, a normal developing system and a reversal developing system are known. For example, in the case of the reversal developing system, as shown in FIG. 5A, a developing bias voltage VB is set between a charging potential V0 and an electrostatic latent image section potential (remaining potential) VR formed by the image exposure, and toner is adhered onto a contrast potential |VB-VR| between the developing bias potential VB and the remaining potential VR, and thereby a toner image having a predetermined density, for example, as shown in FIG. 5B, is obtained.
Further, when the toner image shown in FIG. 5B, is desired to express apparently as a light image, the following operations are conducted: a drive period of light to form the electrostatic latent image is finely controlled; the electrostatic latent image as shown in FIG. 6A is obtained; and the dotted toner image, for example, as shown in FIG. 6B is obtained by adhering the toner onto the latent image; thereby, the amount of the adhering toner per unit area is decreased so as to obtain the low density toner image; or as shown in FIG. 7, the developing bias potential VB is lowered so as to control the contrast potential |VB-VR| to be decreased; and the toner image whose density is lower than that of the toner image of FIG. 5B, is obtained by adhering the toner onto the latent image.
Incidentally, the toner image obtained by the method of FIGS. 5A and 6B is seen apparently to be light in the density, however, since the contrast potential |VB-VR| itself is the same value as in the case of FIGS. 5A and 5B, the density of each point d forming the dot is printed by the dark density same as the toner image in FIG. 5B. Further, although the density of the toner image obtained by the method of FIG. 7 is expressed to be lighter than the toner image in FIG. 5B, conventionally, in this type of printing apparatus, the range of the light and shade of the printing density is generally set to a range of the printing density of 1.0 D-1.4 D.
Recently, also in the printing apparatus using the electrophotographic process, the printing apparatus having the color printing function comes into general use, and the electrophotographic type printing apparatus is developing to conduct the dropout color printing (refer to JP-A-11-78208). Herein, the dropout color means a color, as described in JP-A-11-78208, used for a printed portion by a light color which is often seen in a portion such as an entry frame, note and so on for a payment sheet of the bank, test answer-sheet (mark sheet) and the like. Although an area printed by such a dropout color can be distinguished by the naked eye, it can not be distinguished by an optical reading apparatus having a predetermined wavelength.
In order to realize the dropout color printing, it is necessary that the printing can be conducted in the printing density lower than 0.7 D. Therefore, since the conventional printing apparatus conducts a light and shade adjustment of the printing density between 1.0 D-1.4 D, the dropout color printing can not be conducted.
The object of the present invention is to provide an electrophotographic printing apparatus in which the light and shade adjustment of the printing density can be conducted in the broad range, and which can also cope with the dropout color printing.
The above object can be attained by an electrophotographic printing apparatus that includes a charging means for charging a photoreceptor to a predetermined charging potential |V0| and an exposure means for exposing the charged photoreceptor and recording an electrostatic latent image having an electrostatic latent image section potential |VR| which is lower than the charging potential, on the photoreceptor.
A developing means is provided, having a developing roller on which a developing bias potential |VB| is applied, which is set between the charging potential and the electrostatic latent image section potential, and which adheres a powder color toner held on the developing roller to a portion of the charging potential when a developing method is a normal developing method and to a portion of the electrostatic latent image section potential when the developing method is a reversal developing method, and forms a toner image on the photoreceptor. A printing density control means changes a printing density within a range from a dropout color to a non-dropout color.